消息來源：ScienceNews
Fluorine atoms (illustrated in green) squeeze a hydrogen atom (orange) between them, when dissolved in water (red and silver). Researchers used infrared laser light (red lines) to study the chemical bond that formed (branching blue lines), which acts like a hybrid between a hydrogen bond and a covalent bond.DESIGNED BY ELLA MARU STUDIO
2021年1月7日，艾米莉·康諾弗（Emily Conover）在《科學新聞》（Science News）網站發布的消息，當氟原子（上圖中綠色）溶於水（紅色和銀色）時，它們之間的氫原子（橙色）會受到擠壓。研究人員使用紅外雷射研究形成的化學鍵（分支藍線），其作用類似於氫鍵和共價鍵之間的雜化。
這種奇怪的化學鍵的作用就像氫鍵和共價鍵的混搭，被氟夾在中間的氫原子表現出化學上的怪癖
全世界的化學專業學生都熟悉共價鍵和氫鍵。現在的一項研究揭示了一種奇怪的結合，就像兩者的混合體一樣。化學家發表在2021年1月8日的《科學》（Science）雜誌上報告說，其性質引發了有關如何定義化學鍵的如何定義化學鍵的(how chemical bonds are defined)。（DOI: 10.1126/science.abe1951）
氫鍵通常被認為是弱的電吸引力，而不是真正的化學鍵。另一方面，共價鍵是強化學鍵，可將分子內的原子結合在一起，是原子之間共享電子而產生的。現在，研究人員報告說，氫鍵的異常強大實際上是一種雜化物，因為它涉及共享電子，從而模糊了氫鍵和共價鍵之間的區別。
芝加哥大學（University of Chicago）的化學家安德烈·託克馬科夫說：「我們對化學鍵的理解以及我們對它的教導方式非常清楚，黑白分明。」然而，這項新研究表明「實際上化學鍵本身就是一種連續統一體。」即既不同於共價鍵也不同於氫鍵的雜化鍵。
託克馬科夫和他的同事通過觀察水中稱為二氟離子的原子團來表徵雜化鍵，該原子團由夾在一對氟原子之間的單個氫原子組成（F—H—F）。按照傳統觀點，氫原子通過共價鍵與一個氟鍵合併且通過氫鍵與另一個氟鍵合（F—HF）。
研究人員使用紅外光使二氟離子振動，並測量氫原子的響應，揭示了氫原子振動的一系列能級。對於典型的氫鍵，這些能級之間的間距會隨著原子進一步爬上高能級而減小。但是，研究人員發現間距增加了。這一行為表明氫原子在兩個氟原子之間平等共享，不是通過共價鍵與一個氟原子緊密結合，而是通過典型的氫鍵更鬆散地與另一個氟原子結合（F—HF）。在這種安排中，「共價鍵和氫鍵之間的差異被消除，不再有意義，」研究合著者、芝加哥大學化學家的博格丹·德雷卡（Bogdan Dereka）說。
計算機計算表明，這種行為取決於兩個氟原子之間的距離。當氟原子彼此靠近時，它們之間的氫被擠壓，正常的氫鍵變得更強，直到所有三個原子開始以共價鍵的形式共享電子，形成了一個單鏈，研究人員稱之為氫介導的化學鍵（hydrogen-mediated chemical bond）。對於距離較遠的氟原子，仍然適用具有不同共價鍵和氫鍵的常規描述。
研究人員得出結論，氫介導的化學鍵不能被描述為純氫鍵或純共價鍵。「這真是兩者的混合體，」德國美因茨馬普高分子研究所（Max Planck Institute for Polymer Research in Mainz, Germany）的化學家米莎·波恩（Mischa Bonn）說。米莎·波恩作為一名合作者，也有一篇相關研究在《科學》（Science）雜誌網站發表。（DOI: 10.1126/science.abf3543）
氫鍵存在於多種物質中，最著名的是在水中。沒有氫鍵，室溫下的水將是氣體而不是液體。雖然水中的大多數氫鍵都很弱，但在含有過量氫離子的水中卻可以形成類似於二氟離子中的強氫鍵。兩個水分子可以將氫離子夾在中間，形成所謂的曾德爾離子（Zundel ion），其中兩個水分子之間均等地共享氫離子。德國柏林的馬克斯·玻恩非線性光學和短脈衝光譜學研究所的化學家埃裡克·尼博林（Erik Nibbering）說，「一切都很好。」新的研究結果順應了曾德爾離子的行為。埃裡克·尼博林等人，2017年也曾經在《科學》（Science）雜誌上發表過有關曾德爾離子的研究。（DOI: 10.1126/science.aan5144）
強氫鍵被認為在運輸氫離子中發揮作用，這一過程對於包括電池和燃料電池等技術在內的各種生物機制至關重要。因此，更好地了解這些鍵可以揭示各種影響。
新的觀察結果會影響科學家如何理解化學基本原理。波恩（Bonn）說：「這觸及了我們對化學鍵是什麼的基本理解。」
對化學鍵的新發現也引發了關於什麼是分子的問題。通過共價鍵連接的原子被視為單個分子的一部分，而通過氫鍵連接的原子可以保留為單獨的實體。因此，兩者之間處於邊緣狀態的問題提出了一個問題：「什麼時候從兩個分子變成一個分子？」 託克馬科夫說。
原文：
Chemistry students the world over are familiar with covalent bonds and hydrogen bonds. Now a study has revealed a strange variety of bond that acts like a hybrid of the two. Its properties raise questions about how chemical bonds are defined, chemists report in the Jan. 8 Science.Hydrogen bonds are typically thought of as weak electrical attractions rather than true chemical bonds. Covalent bonds, on the other hand, are strong chemical bonds that hold together atoms within a molecule and result from electrons being shared among atoms. Now, researchers report that an unusually strong variety of hydrogen bond is in fact a hybrid, as it involves shared electrons, blurring the distinction between hydrogen and covalent bonds.「Our understanding of chemical bonding, the way we teach it, is very much black and white,」 says chemist Andrei Tokmakoff of the University of Chicago. The new study shows that 「there’s actually a continuum.」Tokmakoff and colleagues characterized the hybrid bond by observing groups of atoms called bifluoride ions, consisting of a single hydrogen atom sandwiched between a pair of fluorine atoms, in water. According to conventional wisdom, the hydrogen atom is bound to one fluorine by a covalent bond and to the other fluorine by a hydrogen bond.The researchers used infrared light to set bifluoride ions vibrating and measured the hydrogen atoms』 response, revealing a series of energy levels at which the hydrogen atoms vibrated. For a typical hydrogen bond, the spacing between those energy levels would decrease as the atom climbed further up the energy ladder. But instead, the researchers found that the spacing increased. This behavior indicated that the hydrogen atom was shared between the two fluorine atoms equally, rather than being closely bound to one fluorine atom by a covalent bond and more loosely bound by a typical hydrogen bond to the other. In that arrangement, 「the difference between the covalent and [hydrogen] bond is erased and is no longer meaningful,」 says study coauthor Bogdan Dereka, a chemist also at the University of Chicago.Computer calculations showed that this behavior is dependent on the distance between the two fluorine atoms. As the fluorine atoms move closer to each other, squeezing the hydrogen between them, the normal hydrogen bond becomes stronger, until all three atoms begin sharing electrons as in a covalent bond, forming a single link that the researchers call a hydrogen-mediated chemical bond. For fluorine atoms that are farther apart, the conventional description, with distinct covalent and hydrogen bonds, still applies.The hydrogen-mediated chemical bond can’t be described as either a pure hydrogen bond or a pure covalent bond, the researchers conclude. 「It’s really some hybrid of the two,」 says chemist Mischa Bonn of the Max Planck Institute for Polymer Research in Mainz, Germany, who coauthored a perspective piece on the study, also published in Science.Hydrogen bonds occur in a variety of substances, most famously in water. Without hydrogen bonds, water at room temperature would be a gas instead of a liquid. While most hydrogen bonds in water are weak, strong hydrogen bonds similar to the ones found in the bifluoride ions can form in water that contains excess hydrogen ions. Two water molecules can sandwich a hydrogen ion, creating what’s called a Zundel ion, in which the hydrogen ion is equally shared between the two water molecules. The new results echo the Zundel ion’s behavior, says chemist Erik Nibbering of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin, who coauthored a 2017 paper in Science on the Zundel ion. 「It all fits nicely.」Strong hydrogen bonds are thought to play a role in transporting hydrogen ions, a process crucial for a variety of biological mechanisms including powering cells and for technologies such as fuel cells. So better understanding these bonds could shed light on a variety of effects.And the new observation has implications for how scientists understand basic principles of chemistry. 「It touches on our fundamental understanding of what a chemical bond is,」 Bonn says.That newfound understanding of chemical bonding also raises questions about what qualifies as a molecule. Atoms connected by covalent bonds are considered part of a single molecule, while those connected by hydrogen bonds can remain separate entities. So bonds in limbo between the two raise the question, 「when do you go from two molecules to one molecule?」 Tokmakoff says.免責聲明：本平臺部分內容轉載於網絡，目的在於傳遞更多信息，並不代表本平臺贊同其觀點和對其真實性負責；若有侵犯您的權益或其他不適宜之處，請聯繫我們，並提供相關證明，本平臺將及時處理。郵箱：liuxin@hgxci.org.cn；微信：Eli29526